Valve actuators are used to operate valves and are manufactured in numerous shapes, sizes, forms and have a wide variety of utilities. It is common for an operator of a valve actuator to want to know the specific position of a valve. Absolute encoders and incremental encoders have been utilized with valve actuators to determine the position of a valve. The encoders monitor the position of the valve actuator to determine the corresponding position of a valve.
Absolute encoders utilize a unique signature for each position of a valve actuator. Absolute encoders often comprise either a single disc or multiple discs that are rotated as a valve actuator moves to different positions. The single or multiple discs of an absolute encoder have markings, different combinations of which provide a unique signature for each position of a valve actuator. This unique signature can be analyzed at any time to determine the position of the valve actuator.
Incremental encoders, on the other hand, do not have a unique signature for each position of the valve actuator. Instead, incremental encoders monitor changes in the valve actuator relative to an arbitrary starting point, such as the fully closed position of a valve. An incremental encoder, also referred to as a relative encoder, may be a single disc with a series of duplicate markings around the edge of the disc. As the disc is rotated, each time one of the marks passes a point, a change in position is recorded. For example, if a disc had one hundred marks around the edge of the disc, the disc could be rotated multiple times and so that several hundred marks could be counted to indicate changes in position. As long as the marks are recorded in the memory of a computer, then the valve actuator position is known and can be indicated by on-board electronics using status contacts, data bits in user readable data registers, or alpha-numeric displays.
Valve actuators will often have a handwheel, as well, in case the electric motor should either fail or there should be a power failure, so that manual operation of the valve is available. However, a loss of power could result in an erroneous position indication if the handwheel is moved while the power is down and there is no backup or redundant power supply. After the handwheel is moved, the lack of updating can result in a valve actuator indicating one position, when in fact, the valve is in a different position. Actuators using absolute encoders automatically correct the position indication as soon as power is restored. Actuators using incremental encoders must have a backup or redundant power supply to allow them to avoid total loss of position knowledge during a power outage. For this case, when power is lost, the incremental encoder either reverts to its startup position, or its current count is saved to a non-volatile memory location in the on-board electronics. If the handwheel is moved on an incremental encoder system, the electronics will continue to track the position as long as the backup or redundant supply remains alive. If, however, the backup or redundant power supply dies (ex: dead battery), then the incremental encoder based actuator system will lose its proper sense of position if the user moves the handwheel after the backup or redundant power supply dies. Incorrect position information could, in turn, lead to valve damage and incorrect operation of a process controlled by the valve. Either absolute or incremental encoder based systems can maintain proper position information on loss of normal power, as long as there exists a backup or redundant power supply. An absolute encoding system may lose its ability to indicate the proper position during a total power loss, but it will immediately recover on the restoration of power. The incremental system, on the other hand, will need to be recalibrated after a total power loss if the user has also moved the handwheel during the power outage. Users are typically reticent to move a valve that is operating in an active process just to recalibrate the position sensor. To do so, even on a single valve, often requires a complete plant shut down.
The sensing of position information from either an absolute or an incremental encoder requires electrical power. Additionally, when an incremental encoder is used, there is an additional power requirement for storing position information in a memory. Absolute encoders do not require the storage of position information since the information can be readily ascertained from the absolute encoder. Valve actuators that utilize incremental encoders require battery back-up to maintain position information in the event of a power failure. For both absolute and incremental encoders, a back-up power source is necessary to display position information during a primary power failure.
There is a need in the art for electronically displaying updated position information during manual operation of a valve actuator without the need for a back-up battery. A need exists in the art for powering the other electronic components of a valve actuator. Additionally, there is a need in the art for providing electrical power to hydraulic and pneumatic valve actuators without the need for supplying electricity from an external source.